U.S. patent number 4,032,563 [Application Number 05/609,193] was granted by the patent office on 1977-06-28 for process for the recovery of high purity diesters of terephthalic or isophthalic acids.
This patent grant is currently assigned to Standard Oil Company. Invention is credited to Jon J. Harper, Antonio E. Navarrete, Richard J. Thomas.
United States Patent |
4,032,563 |
Harper , et al. |
June 28, 1977 |
Process for the recovery of high purity diesters of terephthalic or
isophthalic acids
Abstract
A process is disclosed for producing the dimethyl esters of
aromatic dicarboxylic acids wherein the acid is esterified with a
low molecular weight alcohol, the product is crystallized and
separated to isolate a mass of the diester crystals and recover a
mother liquor. An improvement in such process resides in the
thermal oxidation with molecular oxygen of a mother liquor material
to produce a mono methylphthalate from either p-methyl toluate or
methyl-4-carboxybenzaldehyde. A portion of the oxidized mother
liquor is recycled to the esterification zone wherein the mono
methylphthalate can be esterified to the dimethyl ester while
another portion of the oxidized mother liquor stream is passed to
an evaporation treatment zone, preferably a wiped film evaporator,
wherein overhead material is separated from a high boiling bottoms
material and recycled to the esterification reaction zone. The high
boiling bottoms stream from the evaporation treatment zone is
purged from the system thereby removing from the overall process
high boiling organics including ash.
Inventors: |
Harper; Jon J. (Naperville,
IL), Navarrete; Antonio E. (Downers Grove, IL), Thomas;
Richard J. (Huntington Beach, CA) |
Assignee: |
Standard Oil Company (Chicago,
IL)
|
Family
ID: |
24439732 |
Appl.
No.: |
05/609,193 |
Filed: |
September 2, 1975 |
Current U.S.
Class: |
560/98; 560/78;
560/77 |
Current CPC
Class: |
C07C
67/39 (20130101); C07C 69/82 (20130101); C07C
67/39 (20130101); C07C 69/82 (20130101) |
Current International
Class: |
C07C
69/00 (20060101); C07C 69/82 (20060101); C07C
069/80 (); C07C 069/82 () |
Field of
Search: |
;260/475B,475R,475 |
Foreign Patent Documents
|
|
|
|
|
|
|
65-15061 |
|
Jul 1965 |
|
JA |
|
978,172 |
|
Dec 1964 |
|
UK |
|
Primary Examiner: Myers; Jane S.
Attorney, Agent or Firm: Sloat; Robert E. Gilkes; Arthur G.
McClain; William T.
Claims
We claim as our invention:
1. In a process for the production of a lower alkyl diester of a
phenylene dicarboxylic acid in an esterification zone wherein
reaction product from said zone is cooled thus rendering a mass of
lower alkyl diester and mother liquor containing impurities
characterized as high boiling organics and methyl esters of
carboxybenzaldehyde and toluic acid, wherein an improvement
comprises:
(1) Oxidation treatment of mother liquor with molecular oxygen at a
temperature in the range of from about 180.degree. to about
250.degree. C. to form a product termed oxidized mother liquor and
return of part of said oxidized mother liquor to the esterification
zone;
(2) Evaporation treatment of another part of said oxidized mother
liquor to effect production of low boiling overhead product and
high boiling bottoms product containing said high boiling
organics;
(3) Return of overhead product to the esterification zone and
removal of high boiling bottoms product from the process.
2. The process of claim 1 wherein said dicarboxylic acid is
terephthalic acid.
3. The process of claim 1 wherein said oxidation treatment includes
contacting said mother liquor with air.
4. The process of claim 3 wherein said oxidation treatment is
performed as essentially thermal oxidation.
5. The process of claim 1 wherein less than about 98 weight percent
of the oxidized mother liquor is passed directly to the
esterification zone and the remaining oxidized mother liquor is
passed to said evaporation treatment.
6. The process of claim 1 wherein the oxidation treatment comprises
conversion of methyl esters of carboxybenzaldehyde and toluic acid
to methyl esters of phenylene carboxylic acid.
7. The process of claim 1 wherein essentially all the mother liquor
is subjected to oxidation treatment.
8. The process of claim 1 wherein said evaporation treatment
comprises contacting oxidized mother liquor with a hot surface
thereby causing overhead product to be vaporized and removing from
the surface the high boiling bottoms product.
9. The process of claim 8 wherein the evaporation treatment is
effected in a wiped film evaporator.
10. The process of claim 1 wherein essentially all the mother
liquor is subjected to oxidation treatment and from about 85 to
about 95 weight percent of said oxidized mother liquor is passed as
recycle directly to said esterification zone, with the remaining
oxidized mother liquor passed to said evaporation treatment.
11. The process of claim 10 wherein the dicarboxylic acid is
terephthalic acid.
12. The process of claim 10 wherein said oxidation treatment is
effected by contacting mother liquor with air.
13. In a process for the production of methyl esters from phenylene
dicarboxylic acid and alcohol in an esterification zone wherein
reaction product from said zone is cooled thus rendering a mass of
dimethyl ester crystals separable from remaining mother liquor
containing impurities characterized as high boiling organics
(including bis(carbomethoxy) benzylbenzoate) and methyl esters of
carboxybenzaldehyde and toluic acid, wherein mother liquor is
stripped of light ends including water and unreacted alcohol in a
stripping zone yielding a mother liquor stripper bottoms, wherein
an improvement comprises:
(1) Oxidation of substantially all the mother liquor stripper
bottoms in an oxidation zone at oxidizing conditions including the
presence of molecular oxygen to effect the conversion of the methyl
esters of carboxybenzaldehyde and toluic acid to the mono methyl
esters of phenylene dicarboxylic acid producing an oxidation zone
effluent termed oxidized mother liquor stripper bottoms;
(2) Passing at least 75 weight percent of said oxidized mother
liquor stripper bottoms as recycle directly to the esterification
zone;
(3) Passing the remaining oxidized mother liquor stripper bottoms
to an evaporation zone to effect production of low boiling overhead
product including said mono methyl esters of phenylene dicarboxylic
acid and high boiling bottoms product including said high boiling
organics;
(4) Return of substantially all said overhead product to said
esterification zone; and
(5) Recovery and purging from the process high boiling bottoms
product.
14. The process of claim 13 wherein said dicarboxylic acid is
terephthalic acid.
15. The process of claim 13 wherein said oxygen is present in
air.
16. The process of claim 13 wherein said oxidation conditions
include temperatures within the range of from about 190.degree. C.
to about 250.degree. C.
17. The process of claim 16 wherein said temperatures are within
the range of from about 210.degree. C. to about 230.degree. C.
18. The process of claim 13 wherein at least 85 weight percent of
said oxidized mother liquor stripper bottoms is passed to said
esterification zone with the remaining oxidized mother liquor
stripper bottoms passed to said evaporation zone.
19. The process of claim 18 wherein about 90 weight percent of said
oxidized mother liquor stripper bottoms is passed to said
esterification zone with the remaining oxidized mother liquor
stripper bottoms passed to said evaporation zone.
20. The process of claim 18 wherein the high boiling bottoms
product from said evaporation zone comprises from about a few up to
about 40 weight percent of the oxidized mother liquor stripper
bottoms fed to said evaporation zone.
21. The process of claim 20 wherein the high boiling bottoms
product from said evaporation zone comprises from about a few up to
about 30 weight percent of the oxidized mother liquor stripper
bottoms fed to the evaporation zone.
22. The process of claim 13 in that said evaporation zone effects
the production of overhead and bottoms product by contacting
oxidized mother liquor with a hot surface thereby causing overhead
product to be vaporized and removing from the surface the bottoms
product.
23. The process of claim 22 in that the evaporation treatment is
effected by a wiped film evaporator.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of art to which this invention pertains is purification
of diesters of phenylene dicarboxylic acids, in particular, the
purification of the diesters of terephthalic or isophthalic acids,
by an oxidation treatment of mother liquor in conjunction with
evaporation treatment of the oxidized mother liquor, thereafter a
bottoms stream of high boiling organics and ash is recovered, and a
low boiling overhead material is returned to the esterification
zone.
2. Description of the Prior Art
Relevant prior art includes Japanese Patent Publication No.
15016/65 (Application No. 55066/62). Disclosed in this publication
is an improvement in a process for producing terephthalic or
isophthalic, acid diesters in which material such as (1) a residual
liquid obtained by further distillation or concentration of a
mother liquor which has been separated from the crystallized
diester and/or (2) the low boiling distillate which is obtained
from the distillation of the crystallized diester, is oxidized and
then returned for recirculation into an esterification zone. The
primary impurities described in either the concentrated mother
liquor or the low boiling material removed from the diester
crystals is thought, according to the Japanese Publication, to be a
formylbenzoic acid methyl ester which upon oxidation forms a mono
methyl ester of terephthalic or isophthalic acid.
In the subject Japanese Publication, a portion of the mother liquor
is subjected to a treatment for decolorization by means of
distillation or the like and then delivered directly to the
esterification zone as recycle without oxidation treatment. The
portion of the mother liquor into which terephthalic or isophthalic
acid diester has migrated or has been concentrated contains a
significant amount of impurities (formylbenzoic acid methyl ester)
and is delivered to an oxidation reactor and then returned to the
esterification reaction zone.
SUMMARY OF THE INVENTION
The present invention comprises an improvement in a process for the
production of lower alkyl esters of phenylene dicarboxylic acids in
an esterification zone wherein the effluent from such zone is
cooled thereby forming crystals of diester product and a mother
liquor stream, the latter being recycled to the esterification
zone. The improvement in the process generally comprises an
oxidation treatment of the mother liquor to convert impurities
present in such mother liquor-namely, including in most instances
but not limited to p-methyl toluate, methyl-4-carboxybenzaldehyde
and bis (carbomethoxy) benzylbenzoate - to their respective acid
esters with return of a portion of the oxidized mother liquor to
the esterification zone, and the evaporation treatment of a
remaining portion of said oxidized mother liquor to effect the
production of a low boiling overhead stream which is returned to
the esterification reaction zone and a high boiling bottoms product
which contains high boiling organics, ash and, in some instances,
catalyst which is removed from the processing system by purge.
In a broad embodiment, the present invention resides in a process
for the production of lower alkyl esters of a phenylene
dicarboxylic acid in an esterification zone wherein reaction
product from said zone is cooled, thus rendering a mass of lower
alkyl ester separable from remaining mother liquor containing
impurities characterized as high boiling organics, ash and methyl
esters of carboxybenzaldehyde and toluic acid wherein an
improvement comprises:
(1) oxidation treatment of mother liquor to form a product termed
oxidized mother liquor and return of part of said oxidized mother
liquor to the esterification zone;
(2) evaporation treatment of another part of said oxidized mother
liquor to effect production of low boiling overhead and a high
boiling bottoms product containing said high boiling organics and
ash;
(3) return of overhead product to the esterification zone with
removal of high boiling bottoms product of the evaporation
treatment from the process.
These and other objects and embodiments of the invention will be
easily ascertained from a further reading of the specification and
attached claims.
BRIEF DESCRIPTION OF THE DRAWING
The attached DRAWING shows, in a brief description, an
esterification zone 1 which is connected to a cooler 2, a flash
zone 3, an oxidation zone 4, and an evaporation zone 5 associated
with an overhead condenser 6. These specific units in the overall
process effect the production of a lower alkyl ester of a phenylene
dicarboxylic acid from the acid and a lower alkyl alcohol such as
methanol.
More particularly, esterification zone 1 has a feed line 7 passing
into it wherein the phenylene dicarboxylic acid, typically
terephthalic or isophthalic acid, along with catalyst and a lower
alkyl alcohol, typically methanol, are passed to be esterified for
the production of the diester. In the esterification zone there is
a suitable contacting at pressure and temperature conditions to
effect the conversion of the acid to the ester. Associated
separation, temperature and pressure control means and contacting
apparatus are not shown for simplicity of process description. Also
shown passing into the esterification zone 1 is an oxidized mother
liquor stream 16 which is directly recycled from the oxidation zone
4 and a low boiling overhead product which is derived from
evaporation zone 5 which passes to the esterification zone via line
21. The overall description of these streams 16 and 21 will be
described in more detail below.
Stream 8 which is removed from the esterification zone 1 typically
will contain the lower alkyl esters produced from the phenylene
dicarboxylic acid along with unconverted alcohol, the methyl esters
of carboxybenzaldehyde and toluic acid, high boiling organics, ash
and in some instances entrained catalyst. This stream is passed via
line 8 into cooler 2 which can function to remove an essentially
purified lower alkyl diester of the acid fed to the esterification
zone by crystallization and separation using either filtration or
centrifugation methods. The recovered crystalline diester passing
through line 9 may be sent to other processing wherein light end
materials are removed and either returned to the esterification
zone, removed from the process, or in some instances passed to an
oxidation zone. Line 10 which leaves the cooler 2 is typically
referred to as a mother liquor stream and contains uncrystallized
diesters, unreacted lower alcohols, some entrained catalyst from
the esterification zone, materials referred to as the methyl esters
of carboxybenzaldehyde and toluic acid, namely,
methyl-4-carboxybenzaldehyde and p-methyl toluate respectively, and
high boiling organics including in some instances bis
(carbomethoxy) benzylbenzoate. The latter three materials, if not
subjected to further treatment such as oxidation, will eventually
either have to be removed from the system or allowed to contribute
to excessive recycling greatly reducing the overall efficiency of
the process. It is because of these particular materials that the
present oxidation of essentially all of the mother liquor in a
preferred instance is performed converting these materials to the
mono methyl esters of the dicarboxylic acid.
The above-described mother liquor stream is passed via line 10 into
a flash zone 3 wherein a portion of material is flashed off. The
overhead product removed via line 11 generally comprises water and
the lower alkyl alcohol plus light ester by-products which are
removed from the process. It is desired to remove these low end or
light boiling materials in order to eliminate the possibility of
explosions within the processing sequence since these materials
when contacted with an oxygen atmosphere can form explosive
mixtures.
The light ends being removed, there is left a stream 12 which is
removed from the flash zone 3. This stream is typically referred to
as a mother liquor stripper bottoms stream and except for the
material removed in the flash zone 3, is essentially the same as
the mother liquor stream removed via line 10 from the cooler. In a
preferred instance, essentially all the mother liquor stripper
bottoms stream is passed into the oxidation zone 4. Into the
oxidation zone 4 a stream containing molecular oxygen is passed via
line 14. Removed from the oxidation zone 4 via line 23 is the
unreacted oxygen, by-product water and small portions of lower
alkyl alcohol and light ends either produced in the oxidation zone
or not totally removed in the flash zone 3 and traces of diester.
In instances where enriched air or purified oxygen streams are
used, the effluent line 13 may be treated for recycle to the
oxidation zone 4. Line 22 carries water, light esters and some
dimethyl ester product and generally is recycled to esterification
zone 1.
The product removed from oxidation zone 4 via line 15 represents
material termed oxidized mother liquor. This material is split,
depending upon the upstream operations, into a stream which can be
recycled directly to the esterification zone 1 via line 16 and a
portion which can be passed to the evaporation zone 5. In a
preferred instance, the portion of the oxidized mother liquor
stream not recycled directly to the esterification zone is passed
via line 17 to the evaporation zone 5.
In the evaporation zone an evaporation treatment takes place to
effect the production of a low boiling overhead product which is
recovered in overhead condenser 6 and returned to the
esterification zone 1 via recycle line 21. Line 20, which is
removed from the overhead condenser, contains a very small fraction
of the total material passing through line 19 and comprises light
end materials produced in any of the aforementioned treatment or
reaction zones which is removed from the process. Because of the
nature of the operation in the evaporation zone, line 20 is
generally connected to an ejector and a vacuum is drawn on the
overhead condenser system 6 via line 20.
The other stream produced in the evaporation zone 5 is termed a
high boiling bottoms product which is preferably removed from the
process via line 18. Because of the nature of the material
representing the high boiling bottoms product (high boiling
organics, ash and in some instances esterification zone catalyst),
it is desirable to remove at least a portion or preferably all of
this material from the process through purge.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is primarily concerned with the oxidation of
a mother liquor stream which has been separated from a crystalline
dimethyl ester product produced in an esterification zone and the
subsequent treatment of a portion of the oxidized mother liquor
stream in an evaporation treatment zone. In the evaporation
treatment zone a low boiling overhead product containing diester
precursors is recovered and returned to the esterification reaction
zone. A high boiling bottoms product containing high boiling
organics, ash and in some instances esterification reaction zone
catalyst is also recovered from the evaporation zone which is
removed in a preferred instance as a purge from the overall
processing scheme.
The process into which the above described combination is
incorporated has an overall flow scheme as described in the
description of the drawing above. Therein the esterification zone
can be operated at temperatures well known in the art in order to
induce the esterification of the phenylene dicarboxylic acid
(generally terephthalic or isophthalic acid) with a lower alkyl
alcohol such as methanol thereby forming the dimethyl ester along
with certain other by-product materials. The esterification zone
typically uses very small quantities of catalyst, generally on the
order of around 0.05% catalyst based on the feed to the
esterification zone. Preferably the catalyst is a zinc oxide
powder. The reaction temperatures, pressures and overall flow and
reactor design for the esterification zone are relatively well
known in the art and it is not considered necessary for one having
ordinary skill in the art to elaborate on these particulars. The
same holds for the cooling zone wherein the dimethyl esters are
separated as a solid crystalline product from a mother liquor
stream, since this operation is typically performed by using known
filtration or centrifugation methods. It is not especially critical
to the overall process or the improvement as claimed herein to
select methods, operating temperatures and pressures and detailed
description of the apparatus needed to perform this function.
Likewise, the flash zone which is utilized to remove the alcohol,
water and some light esters from the mother liquor which has been
separated from the dimethyl crystal product should generally be run
at temperature, pressure and overall conditions and in an apparatus
well known in the art for such purposes.
The oxidation reaction zone performs the essential function of
oxidizing certain of the impurities remaining in the mother liquor
before it is returned to the esterification zone or passed to the
evaporation zone as will be described below. Specifically, reaction
conditions within this zone should generally include temperatures
known in the art and generally within the range of about
180.degree. to about 250.degree. C. or higher. More preferred
temperature ranges should be within the range of from about
215.degree. to about 225.degree. C. with an especially preferred
range being around 220.degree. C. It is generally necessary, at
least when operating at pressures of around one atmosphere and
using air as the oxygen-containing medium, that temperatures
greater than about 210.degree. C. be maintained in order to achieve
an over-rich vapor phase since the explosive range for equilibrium
vapor concentrations is explosive within the range of from about
130.degree. to about 202.degree. C. at one atmosphere pressure.
The pressures within the oxidation reaction zone can vary within
the well-known ranges. Primarily in order to reduce the cost of
operating this particular apparatus and associated equipment, it is
generally preferred to operate around atmospheric pressure but this
is not a criticality.
The typical apparatus design for an oxidation reaction zone would
generally be a reaction zone having the oxygen in upflow
arrangement with baffling or impelling mixers present in such zone
in order to adequately contact the mother liquor passed into the
stream with the molecular oxygen passing through the zone.
The primary purpose for the use of the oxidation reaction zone is
to convert certain impurities present in the mother liquor to
dimethyl ester precursors through the mechanism of oxidation. In
particular, the impurities generally treated in the oxidation
reaction zone include, depending on the acid used in the
esterification zone, the methyl esters of toluene and benzaldehyde.
In instances in which terephthalic acid is used as the acid feed to
the esterification zone, these impurities are described as
p-methyltoluate and methyl-4-carboxybenzaldehyde. The other
impurities, namely, the high boiling organics, ashes and in some
instances esterification zone catalyst also are exposed to oxygen
in the oxidation reaction zone and to some extent are converted to
more oxidized components.
The reactions taking place in the oxidation reaction zone include
the oxidation of the aforementioned methyl esters of
carboxybenzaldehyde and toluic acid to their respective methyl
ester acids. The methyl ester acids are generally termed diester
precursors since when they are recycled to the esterification zone
their acid moiety is esterified with alcohol thereby forming the
diester product which can be recovered from the process.
Other precursors include the oxygenation product of bis
(carbomethoxy) benzylbenzoate, specifically 4,4'-dicarboxybenzyl
benzoate. This material boils within the high boiling organic
boiling range and is produced in fairly large quantities when
terephthalic acid feed is used in the esterification zone. Upon
oxidation the bis (carbomethoxy) benzylbenzoate is thought to be
converted to an anhydride at the bridge connecting its two aromatic
moieties. After formation of the anhydride it is believed that it
combines with water or methanol to form additional acids or esters
giving increased product yields.
It is important to oxidize essentially all of the mother liquor to
recover a maximum quantity of diester precursors since they appear
in both the high boiling and low boiling fractions of the mother
liquor. If a portion of the mother liquor is removed prior to
oxidation then it is entirely possible that a reduction in diester
yield will result.
The oxidation conditions effected in the oxidation reaction zone
generally are performed without the use of a catalyst since
applicants have found that suitable conversion of the methyl esters
of benzaldehyde and toluene and the bis (carbomethoxy)
benzylbenzoate can be effected when using only thermal oxidation.
In some instances, however, oxidation may take place with or
without molecular oxygen and with or without the presence of an
oxidation catalyst or promoter. Of course, it is possible that in a
system in which no external catalyst is added to the oxidation
reaction zone, there may be inherent catalytic sites within the
oxidation reaction zone by virtue of the material selected for its
construction or the impurities which reside within such reaction
zone or in the streams passing thereto.
The operating conditions which take place in the evaporation zone
are generally regulated to effect the separation of the oxidized
mother liquor fed to the evaporation zone into a high boiling
bottoms product comprising high boiling organics, ash and in some
instances esterification zone catalyst, and a low boiling overhead
product which comprises in part the oxidized methyl esters of
methyl-4-carboxybenzaldehyde and p-methyl toluate) together with
some diester product and oxidized bis (carbomethoxy)
benzylbenzoate. The low boiling overhead product can be returned to
the esterification zone for conversion of the above described
materials to diester product. The high boiling bottoms product is
recovered as a bottoms product and removed from the process as a
purge stream.
The purge effectively prevents a buildup in any or all of the
processing units of solid materials and/or sludge caused by recycle
of the high boiling organics, ash, and/or entrained esterification
zone catalyst solids. This purge also essentially eliminates loss
of product diester or any of its precursors since these materials
are removed from the high boiling material by the evaporation
treatment and returned to the esterification reactor.
One of the important advantages in utilizing the evaporation zone
in conjunction with the oxidation zone is that the combination
allows the removal of high boiling bottoms material from the
evaporation zone while substantially minimizing the losses of
diester product or diester precursors via the bottoms purge. If a
purge is taken of straight mother liquor, whether or not oxidized,
increased losses of diester product will result since it is present
in the mother liquor. If the mother liquor is not oxidized then
there is a further reduction in diester product yield since there
is no production of methyl ester acid which can be converted to
diesters.
Because of the nature of the high boiling bottoms stream which is
desired to be contacted in the evaporation treatment zone, it is
especially preferred in this zone that separation of the low
boiling overhead product from the high boiling bottom product be
effected by contacting the oxidized mother liquor feed with a hot
surface thereby effecting evaporation of overhead product from a
high boiling bottoms product which typically will remain on the hot
surface. One of the methods of utilizing such a processing sequence
is through incorporation of a wiped film evaporator. In such an
apparatus, the oxidized mother liquor stream fed thereto contacts
the hot walls of the evaporator causing the low boiling overhead
portion to be evaporated. The remaining high boiling fraction in
the form of a solid or semi-solid which remains on the hot walls is
physically scraped or removed from the walls and recovered. Because
the residual product is extremely high in viscosity, it is very
much susceptible to forming heavy solids deposits on the walls of
the hot surface it contacts, the use of a distillation columns as
known in the art is generally not used.
The operating temperatures and pressures of the evaporation
treatment zone can be selected from those fairly well known in the
art and should include temperatures in the range of around
220.degree. C. and pressures, preferably vacuums, generally in the
range of from about 0.04 to about 0.05 kilograms per square
centimeters absolute. In effecting the evaporation treatment of the
portion of the oxidized mother liquor stream passed into the
evaporation zone, an especially preferred operation will allow
essentially 20 wt% of the oxidized mother liquor fed thereto to be
removed from the evaporation zone as a high boiling bottoms product
with about 76 wt% of the feed to the evaporation zone resulting in
a low boiling overhead product which preferably is recycled to the
esterification zone for the production of additional dimethyl
esters. The remaining 4 wt% of the feed to the evaporation zone may
be lost via the vacuum lines when inducing a vacuum into the
overhead section of the evaporation zone. It is entirely possible,
depending upon the operating conditions, catalyst used, specific
feeds to the esterification zone and other operating variables
affecting the production of the dimethyl ester to vary the ratio of
low boiling overhead product to the high boiling bottoms product.
It is especially preferred during steady state operations that the
quantity of high boiling bottoms product removed from the process
by regulated so as to minimize the loss of any valuable products
which are present in this stream. If the evaporation zone is
properly regulated with respect to operating temperatures and
throughputs, the high boiling bottoms product removed from this
stream will contain a minimum quantity of valuable dimethyl esters
and dimethyl ester precursors.
The other important variables which can be regulated to the effect
of the improvement claimed herein resides in the quantity of the
mother liquor which is oxidized. It is preferred that essentially
all of the mother liquor which is directly recycled to the
esterification zone or indirectly recycled to the esterification
zone (via the low boiling overhead product recycled from the
evaporation zone) be oxidized. It is especially preferred that
essentially all of the mother liquor removed from the dimethyl
ester product be oxidized. Of course normal sampling and leakage
will in most instances prevent oxidizing all the mother liquor.
Additionally, the ratio of the oxidized mother liquor which is fed
directly to the esterification zone (via line 16 of the drawing) to
that mother liquor which is indirectly recycled to the
esterification zone (via lines 17, 19 and 21 of the drawing) can be
regulated so as to effect the overall desired advantages described
above. Specifically, when all or essentially all of the mother
liquor is oxidized it is especially preferred that less than 98 wt%
of such oxidized mother liquor be fed directly to the
esterification zone with the remaining portion of the oxidized
mother liquor passed directly to the evaporation zone. At steady
state conditions it is also preferred that greater than 80% of the
oxidized mother liquor be passed directly to the esterification
zone with the remaining portion of the oxidized mother liquor being
indirectly recycled by first being passed to the evaporation zone
for the aforementioned evaporation treatment. Again, in instances
when essentially all of the mother liquor stream is oxidized and
essentially all of the oxidized mother liquor is returned to the
esterification zone either directly or indirectly (through the
evaporation zone) that from about 85-95 wt% of the oxidized mother
liquor material is fed directly to the esterification zone with the
remaining 15-5 wt% of the oxidized mother liquor stream passed
directly to the evaporation zone. It is especially preferred that
approximately 90 wt% of the oxidized mother liquor be passed
directly to the esterification zone with the remaining 10 wt% of
the mother liquor passed to the evaporation zone for separation
into a low boiling overhead product and a high boiling bottoms
product with essentially total recycle of the low boiling overhead
product to the esterification zone.
The effluent stream from the esterification zone described above
contains material termed high boiling organics. This material for
purpose of definition includes organics in the broad sense which
have boiling points above that of methyl trimellitate. In
determining this component a distillation is performed at
194.degree. C. and 12 mm Hg pressure with the material remaining as
bottoms referred to as high boiling organics.
Typically the high boiling organics will contain materials referred
to as ash. The ash component is essentially non-distillable and is
determined by subjecting a sample to be analyzed for ash content to
the following general procedures.
A sample of about 25-35 g. is placed in a platinum dish and ignited
until the sample no longer supports visible combustion. Then the
remaining sample is subjected to oxidation at 300.degree. C. in air
for about one hour. The remaining material (ash) is then calculated
as a percentage of the original sample for ash content.
The high boiling bottoms product from the evaporation treatment
zone will vary in composition depending primarily on the operating
condition in that zone. It will contain a fairly high concentration
of the above defined high boiling organics including ash. The
separation of these materials from dimethyl esters and mono methyl
esters results in the latter also being present in the high boiling
bottoms product from the evaporation treatment zone. A typical high
boiling bottoms product from the evaporation zone will contain
about 20 wt.% of high boiling organics and esters boiling above
monomethyl phthalate, about 44 wt.% monomethyl phthalate with the
remaining 36 wt.% comprising dimethyl ester product.
The following examples are presented to illustrate specific
embodiments of the improvement described herein and are not
necessarily presented so as to unduly limit the scope of the
claims.
EXAMPLE 1
In this example oxidation was performed in a round bottom flask
fitted with a stirrer and a glass inlet tube. A glass wash bottle
was used which contained a fritted glass bottom in order to allow a
better contacting of air with the liquid being oxidized. For a
typical oxidation run about 150 grams of a commercial mother liquor
stripper bottoms material was placed in the glass wash bottle. The
wash bottle was placed in an oil bath which was regulated by
thermostat at either 200.degree. or 220.degree. C. The wash bottle
was purged with nitrogen as it was heated to the given reaction
temperature. When the reaction temperature was obtained, air was
introduced into the reaction vessel at 393 milliliters per minute
(one atmosphere) for an hour. After the hour of oxidation the
contents in the reactor were poured out, allowed to solidify, then
thoroughly ground and analyzed.
The data shown in the following tables illustrate the substantial
reduction via oxidation of both methyl-4-carboxy-benzaldehyde and
p-methyl toluate, each being converted to its respective mono
methyl esters of terephthalic acid.
TABLE 1 ______________________________________ Initial wt. % wt. %
after 60 min. Sample methyl-4-carboxy- oxidation Description
benzaldehyde at 200.degree. C. at 220.degree. C.
______________________________________ 1 3.9 1.59 0.0081 2 3.6 1.13
0.0093 3 3.1 0.013 -- 4 3.5 0.017 -- 5 4.3 0.034 0.0087 6 4.06
0.054 -- 7 3.38 0.012 -- 8 4.42 -- 0.0086 9 3.91 0.014 -- 10 3.68
0.021 -- 11 2.96 0.012 -- 12 3.65 0.007 -- 13 3.57 0.960 0.67 14
4.53 0.004 -- 15 4.60 0.004 0.011
______________________________________
TABLE 2 ______________________________________ wt. % after 60 min.
Sample Initial wt. % oxidation Description p-methyl toluate at
200.degree. C. at 220.degree. C.
______________________________________ 1 1.1 0.38 -- 2 1.0 0.20 --
3 0.59 0.0009 -- 4 0.60 0.012 -- 5 0.85 0.033 -- 6 0.81 0.034 -- 7
0.52 0.001 -- 8 0.66 -- 0.0087 9 0.65 0.001 -- 10 0.66 0.003 -- 11
0.70 -- -- 12 0.83 0.002 -- 13 0.78 0.25 0.29 14 1.04 -- -- 15 0.98
-- -- ______________________________________
EXAMPLE 2
In this example, a mother liquor stripper bottoms stream from a
commercial dimethylterephthalate production unit was oxidized using
the procedure which is generally described for Example 1 except
that the temperature was maintained at 200.degree. C. during the
oxidation step. The material which was oxidized together with the
oxidized product were analyzed to determine the degree of
conversion of the methyl-4-carboxybenzaldehyde and p-methyltoluate
to mono methyl terephthalate. Additionally, high boiling organic
content was measured to determine the extent of reduction of these
materials together with the extent of their conversion to usable
dimethylterephthalate precursors which if fed to the esterification
zone would result in additionally gained product yields of
diester.
The results of the product analysis are shown in Table 3 below and
indicate that essentially all of the p-methyltoluate and
methyl-4-carboxybenzaldehyde were converted to their respective
mono methyl terephthate esters. Additionally, a substantial
reduction in the quantity of high boilers material was realized by
the oxidation procedure which results in an improved product yield
since a portion of these high boiling materials were converted to
diester precursors which when fed back to the esterification zone
result in additional product diester.
TABLE 3 ______________________________________ Analysis Component
Initial Final ______________________________________ P-methyl
toluate, wt. % 0.72 0.015 methyl-4-carboxy benzaldehyde, wt. % 3.08
0.062 High boilers, ppm 14,888 11,524
______________________________________
EXAMPLE 3
In this example a commercially produced mother liquor stripper
bottoms stream was oxidized using conditions and procedures
identical to those described for Examples 1 and 2 above. More
particularly, a stream which contained approximately 63.88 wt.% of
dimethylterephthalate, 22.7 wt.% of mono methyl terephthalate, 0.72
wt.% p-methyl toluate, 3.08 wt.% methyl-4-carboxybenzaldehyde, 2.04
wt.% terephthalic acid and approximately 14,888 ppm of high boiling
organics was oxidized using the aforesaid procedures. After
oxidation the mono methyl terephthalate concentration had increased
3.0 wt.%, the terephthalic acid content increased 0.3 wt.% with
corresponding decreases of the p-methyltoluate and the
methyl-4-carboxybenzaldehyde of 0.7 and 3.02 wt.%, respectively.
Furthermore, there was a noticeable decrease in the concentration
of the high boiling organics from the initial 14,888 ppm
concentration to a post oxidation concentration of 11,524 ppm. Of
important significance in the oxidation procedure was the reduction
in the high boiling organics and in particular, the decrease in the
concentration of bis (carbomethoxy)benzylbenzoate (specifically,
4,4'-dicarboxybenzyl benzoate) from an initial concentration of
6,140 ppm to a post-oxidation concentration of 107 ppm. The
substantial decrease in this component is quite important for its
oxidation product yields diester precursors such as mono methyl
terephthalate or dimethyl terephthalate itself. The results of the
oxidation of the mother liquor stream and in particular, the high
boiling aspects of this stream, are shown for the high boiling
organics only in Table 4 attached.
TABLE 4 ______________________________________ High Boiling
Organics Concentration, ppm (Methyl Esters) Initial Post Oxidation
______________________________________ Dicarboxybiphenyl
Isomers.sup.A 712 811 Dicarboxyindane 12 7 Bis (Carboxyphenyl)
methane 120 107 1,2,3,5-Tetracarboxybenzene 33 33
Unidentified.sup.B 24 6 Carboxybenzophenone 45 39
4,4'-Dicarboxybiphenyl 2,400 2,403 Carboxybenzyl Toluate 51 26 Bis
(4-carboxyphenyl) Methane 16 17 Cis 4-4'-Dicarboxystilbene 62 4
4,4'-Dicarboxybenzophenone 710 676 2,4',5-Tricarboxybiphenyl 2,200
3,471 Tricarboxy biphenyl.sup.C 210 89 Unidentified.sup.B 180 142
Tricarboxydiphenylmethane 11 53 2,6-Dicarboxyfluorenone 60 961
4,4'-Dicarboxybenzyl benzoate 6,140 107
Trans-4,4'-Dicarboxystilbene 230 214 Unidentified.sup.B 1,672 2,358
Total 14,888 11,524 ______________________________________ .sup.A
includes all isomers not possible to specifically identify. .sup.B
structure not known. .sup.C specific isomer not identified.
EXAMPLE 4
In this example samples 1, 2, 3, 4, 5 and 8 of Example 1 were
analyzed for metals content. The metal content is reported in Table
5 below and represents the ppm concentration in the total mother
liquor stripper bottoms stream from a commercial unit. The metals
are present in the high boiling bottom material eventually removed
from the evaporation zone in the process of this invention and are
considered as part of the ash present therein.
Table 5 ______________________________________ Sample Component
content, ppm Total Description Br Co Fe Mn ppm
______________________________________ 1 574 312 77 1592 2555 2 497
250 64 1388 2199 3 368 220 67 1049 1706 4 321 229 112 1370 2032 5
593 258 98 1554 2513 6 568 203 105 1125 2001
______________________________________
EXAMPLE 5
In this example a mother liquor stripper bottoms stream from a
commercial dimethyl terephthalate production unit was treated in an
evaporation zone to illustrate the separation of low boiling
overhead material from a high boiling bottoms material which
contained high boiling organics.
The evaporation treatment zone in this example was a wiped film
evaporator having about 0.125 square meters surface area. The
evaporator had an 82 mm inside diameter with a single heating
section about 500 mm long. The rotor in the evaporator had four
blades and during the run was maintained at a constant 2,000 RPM
during operation. The blades of the rotor were designed to have a
close tolerance to the walls of the evaporator to scrape the high
boiling bottoms from the wall for eventual recovery.
The mother liquor stripper bottoms was fed to an agitated jacketed
feed holding tank where it was maintained in a liquid state. The
resulting melt was fed via a suitable pumping means to the wiped
film evaporator. A high boiling bottoms material from the
evaporator was collected in a bottom collection drum while the low
boiling overhead material was passed through a down flow glycol
cooled heat exchanger where condensation occurred. The distillate
thus obtained was collected in a recovery drum while uncondensed
vapors were passed through a hot water cooled cold trap and from
there to a series of jets which generated a vacuum on the system.
Periodic sampling of the overhead and bottoms streams was performed
to determine the separation effected by the evaporation treatment
zone.
A run of collected bottoms from previous runs was also processed.
In this case the feed to the evaporation zone was a high boiling
bottoms product.
Runs 1-3 in Table 6 below were on mother liquor stripper bottoms
material while run 4 used a bottoms product as feed illustrating
the effects of reconstituting a high boiling bottoms material.
Table 6 ______________________________________ Run No. Parameters 1
2 3 4 ______________________________________ Feed rate, Kg/hr. 34.8
23.4 28.8 16.2 Distillate rate, Kg/hr. 22.2 17.5 25.4 4.2 Bottoms
rate, Kg/hr. 13.4 5.22 3.24 5.9 Heating medium temperature,
.degree. C. 255 259 265 265 Cooling medium temperature, .degree. C.
145 145 145 145 Vapor temperature, .degree. C. 196 190 186 199
Bottoms temperature, .degree. C. 203 200 200 204 Pressure at exit,
mm Hg. obs. 58.sup.(1) 35 30 25 Bottoms/Feed .times. 100, % 38.5
22.3 11.3 36.4 ______________________________________
The bottoms streams produced for runs 1, 2, 3 and 4 above were
analyzed for metals content by x-ray fluorescence and for ash
content and are reported in Table 7 below.
Table 7 ______________________________________ Component
Description Steam Mang- Description Ash Bromine Cobalt Iron anese
Zinc ______________________________________ Run 1 Overhead, ppm 40
74 3 -- 3 3 Bottoms, ppm 14,900 510 250 -- 920 14,000 Run 2
Overhead, ppm 11 79 3 -- 3 3 Bottoms, ppm 22,400 700 380 -- 1,400
1,400 Run 3 Overhead, ppm 19 90 3 -- 3 3 Bottoms, ppm 37,700 800
520 -- 2,200 2,300 Run 4 Overhead, ppm 24 109 3 6 3 3 Bottoms, ppm
62,900 3,600 1,500 2,100 5,300 41,000
______________________________________
EXAMPLE 6
In this example there is illustrated the overall claimed process
including the oxidation of essentially all the mother liquor
stripper bottoms together with evaporation treatment of a slip
stream from the oxidation zone. In order to simplify the
illustration certain operations such as heat exchange, pressure,
level and flow control have been omitted.
The oxidation zone is maintained at about 220.degree. C. and a
pressure of about 1.49 Kg/cm.sup.2 absolute. Air is fed up-flow to
the oxidation zone which performs its function in the absence of
added oxidation catalyst.
The wiped film evaporation zone is maintained at around 220.degree.
C. on the film side of the unit. The feed to this unit is at about
200.degree. C. and 1.010 Kg/cm.sup.2 absolute pressure. Essentially
all of the high boiling bottoms from the evaporation zone is purged
from the system.
Tables 8, 9 and 10 below show the concentrations and conditions of
relevant components and processing streams with reference to the
drawing described above.
The light esters listed in the tables generally comprise
methylbenzoate while the other esters generally comprise esters
boiling above monomethyl terephthalate. The high boiling organics
as used in the claims and specification generally include the last
two listed components in such tables.
Table 8 ______________________________________ Steam Description 12
13 14 ______________________________________ Component, Kh/hr. Air
-- 28.10 31.00 Methanol 2.80 0.91 -- Water 3.10 13.20 -- Light
Esters 13.30 -- -- P-methyl toluate 3.90 -- --
methyl-4-carboxybenzaldehyde 16.80 -- -- Dimethyl terephthalate
1477.70 -- -- Dimethyl iso or ortho phthalate 74.10 -- -- Mono
methyl terephthalate 392.70 -- -- Heavy esters 34.00 -- -- High
boiling components and ash 70.00 -- -- Total 2088.40 42.20 31.00
Pressure, Kg/cm.sup.2 /abs. -- 1.03 1.49 Temperature, .degree. C.
220 74 220 ______________________________________
Table 9 ______________________________________ Steam Description 16
17 18 ______________________________________ Component, Kh/hr. Air
-- -- -- Methanol -- -- -- Water -- -- -- Light Esters 9.00 1.00 --
P-methyl toluate -- -- -- Methyl-4-Carboxybenzaldehyde 16.80 -- --
Dimethyl terephthalate 1291.60 143.40 13.50 Dimethyl iso or ortho
phthalate 64.60 7.20 0.70 Mono methyl terephthalate 370.60 41.20
17.60 Heavy esters 30.60 3.40 2.00 High boiling components and ash
63.00 7.00 6.70 Total 1829.4 203.20 40.50 Pressure, Kg/cm.sup.2
/abs. -- -- -- Temperature, .degree. C. 220 220 200
______________________________________
Table 10 ______________________________________ Steam Description
20 21 22 ______________________________________ Component, Kg./hr.
Air 1.00 -- -- Methanol -- -- -- Water -- -- 156.90 Light Esters
0.40 -- 3.30 P-methyl toluate -- -- -- methyl-4-carboxybenzaldehyde
-- -- -- Dimethyl terephthalate 6.50 123.40 42.70 Dimethyl iso or
ortho phthalate 0.30 6.20 2.30 Mono methyl terephthala 0.20 23.40
4.00 Heavy esters -- 1.40 -- High boiling components and ash --
0.30 -- Total 8.40 155.30 209.20 Pressure, Kg/cm.sup.2 abs. 0.04
0.04 -- Temperature, .degree. C. 154 154 74
______________________________________
* * * * *